There is a continuing evolution towards flash memory devices with higher and higher memory capacities, reached by reducing the size of memory cells. As known to those skilled in the art, the limitations in scaling down integrated devices, besides being due to technological limitations of the fabrication processes, may also be determined by criticalities that negatively affect reliability of the produced devices and therefore reduce the yield of the fabrication process.
Usually, fabricated devices are tested in relation to known critical mechanisms which are accentuated by the scaling down process. This is done by subjecting the devices to specially designed testing cycles. During these testing cycles one of the factors for enhancing critical aspects is the temperature. Increased temperature generally tends to worsen or accelerate the failure mechanisms of those devices that contain elements or cells operating at the limit of their reliability.
It has been recently discovered a new failure mechanism of flash memories which apparently is induced exclusively by cyclings, that is, by the number of erasing and programming cycles to which the memory device is subjected. The new failure manifests itself as a loss of charge of single cells during prolonged tests of data retention of data programmed at a low temperature (for example, at ambient temperature).
Although the probability of this phenomenon, in terms of number of lost bits is very low (ranging from 10.sup.-8 to 10.sup.-9), its occurrence implies the failure of memories commonly having a capacity of many megabytes. This results in missing the specified failure targets required by many applications for these devices.
On the other hand, this type of failure hardly lends itself to a preventive screening at the level of the testing process because the failure mechanism is induced by the number of cycles and the loss of charge may not otherwise be accelerated. This problem has been recently approached at a system level by introducing a dedicated refresh operation of the contents of the programmed memory. However, this does not eliminate the problem, because it requires that the system be permanently powered. A peculiar characteristic of nonvolatile memories should be their capacity of retaining the programmed information, even in absence of power supply.